Chapter 3: Tissues in Action (Biology)

Answer: (iii). Meristematic cells are actively dividing cells: thin cellulose walls, dense cytoplasm, a large prominent nucleus and almost no vacuoles. They are not yet differentiated, so they can keep dividing.

(iii) Thin walls, dense cytoplasm and a large prominent nucleus.

Answer: (ii) Phloem. Phloem carries food (sugars made in the leaves) to other parts of the plant, including the roots. Xylem carries water and minerals upwards from the roots.

(ii) Phloem.

Answer: (iii). A thin layer (e.g. the single layer of squamous epithelium in lung air sacs and blood vessels) gives a short distance for diffusion, so gases, nutrients and wastes can pass across quickly.

(iii) To allow quick exchange of materials across them.

• Straight-leg jump: the knees and ankles stay locked and nearly straight; the hips hardly bend. The jump is low, the push-off comes mostly from the toes, and the landing is hard and jarring, because the joints cannot absorb the shock.
• Normal jump: before take-off the hips, knees and ankles all bend (flex), lowering the body; then they straighten (extend) together, powerfully pushing the body up. On landing they bend again, acting like springs that absorb the impact.

So in the normal jump the joints work together and the muscles (thigh, calf, buttock) get a longer push, giving a higher and safer jump.

In the straight-leg jump the ankle, knee and hip stay almost straight (low jump, hard landing); in the normal jump all three bend and then straighten together, giving more height and a soft, shock-absorbing landing.

Answer: (ii) Hinge joint. The knee (and, for bending up and down, the ankle) moves back and forth in one plane, like a door hinge.

(ii) Hinge joint.

• A: (iii). The assertion is true, but the lung epithelium is a single layer of thin, flat (squamous) cells that speeds up diffusion, so the reason is false.
• B: (i). Both true; plenty of mitochondria (energy) and blood supply (oxygen) let cardiac muscle work non-stop, so R explains A.
• C: (iv). Tendons connect muscle to bone (ligaments connect bone to bone), so A is false; R is true.
• D: (iii). A is true; R is false: in a hinge joint the bone ends fit so as to allow movement in one plane only.

A (iii), B (i), C (iv), D (iii)

(i) The diameter keeps increasing with age, but not at a constant rate: slowly in the first 10 years (4 to 8 cm), fastest between 10 and 20 years (8 to 24 cm, i.e. 1.6 cm per year), and then more steadily (about 0.8 cm per year). The tree grows in girth throughout its life.

(ii) The number of annual rings is equal to the age of the tree (one ring is added every year), and the diameter increases as the number of rings increases. So diameter and annual rings are directly related; counting the rings tells the age of the tree.

(iii) The vascular cambium, a lateral meristem, is responsible for the increase in girth (secondary growth). It lies between the xylem and phloem in the stem (the cork cambium, below the bark, adds the outer protective layers).

Diameter rises with age (fastest in years 10–20); rings = age, so diameter increases with the number of rings; girth is increased by the vascular cambium, a lateral meristem between xylem and phloem.

(i) The bark (cork and the phloem just inside it) protects the tree and carries food. Debarking hampers protection (against water loss, insects, fungi, injury) and the transport of food from the leaves down to the roots through the phloem.

(ii) Further damage would affect the vascular cambium (which makes new xylem and phloem) and the xylem (sapwood) beneath it.

(iii) If the tissues beneath the bark (cambium and xylem) were severely damaged, the upward transport of water and minerals from the roots to the leaves would stop, and the tree could no longer grow in girth. The leaves would wilt and the tree would die.

(iv) Assumptions: that the bark was removed in a complete ring around the trunk (girdling); that the phloem was removed with the bark; that the cambium and xylem were not damaged at first. If only a strip of bark (not a full ring) was removed, food could still flow through the remaining bark and the wound might heal, so the tree could survive. If the bark was removed in a full ring, the roots would starve and the tree would slowly die even if the xylem was intact.

(i) Protection and downward food transport (phloem). (ii) The vascular cambium and xylem. (iii) Water and mineral transport upward (and growth in girth). (iv) Assumes the whole ring of bark and phloem was removed; if only a strip was removed, the tree may recover.

Collenchyma. Its living, elongated cells have walls thickened unevenly at the corners (with cellulose and pectin), which gives both support and flexibility, so young stems bend without breaking.

If it were replaced by sclerenchyma (dead cells with thick, lignified walls), the stem would become hard, rigid and stiff. It could not bend with the wind, so strong monsoon winds could snap it. Also, the young stem would not be able to grow and expand easily, since sclerenchyma cells are dead.

Collenchyma gives flexibility; if replaced by sclerenchyma, the stem would be rigid and brittle and could break in strong winds (and could not grow as easily).

(i) Type B cuttings had a node with a bud, which contains meristematic tissue that can divide and grow into a new shoot and roots. Type A cuttings were pieces of the internode only, with no bud, so they could not sprout.

(ii) Type B had at least one node with a bud (eye); type A did not.

(iii) Whether the cuttings sprouted (new shoots and roots appeared), observed over a few weeks; this could be measured by counting the cuttings that sprouted, the days taken, and the height of new shoots.

(iv) Same length and thickness of cuttings, same variety and age of sugarcane, same soil, amount of water, sunlight, temperature and planting depth, and the same number of cuttings of each type.

(i) B had a node with a bud (meristem) that could grow; A was only an internode. (ii) The presence of a node/bud. (iii) Sprouting of shoots and roots (number, time, height). (iv) Same size and variety of cuttings, soil, water, light, temperature, depth and number.

Both are partly right; Rajiv's point is more complete.

• Simple permanent tissues (parenchyma, collenchyma, sclerenchyma) are made of one type of cell, all similar in structure, performing a common function. Rohan's definition fits them.
• Complex permanent tissues (xylem and phloem) are made of more than one type of cell, which may differ in structure, but all work together for one common function. For example, xylem has tracheids, vessels, xylem parenchyma and xylem fibres, all helping to conduct water and minerals; phloem has sieve tubes, companion cells, phloem parenchyma and phloem fibres, all helping to transport food.

So a better definition is: a tissue is a group of cells, similar or different, that work together to perform a particular function.

Simple tissues have one kind of cell (Rohan's definition fits), but complex tissues like xylem and phloem have several kinds of cells working together for one function, so a tissue is a group of cells that work together for a common function.

Sclerenchyma. The husk of the coconut is made of sclerenchyma fibres: long, narrow, dead cells whose walls are thickly deposited with lignin, with almost no space inside. This makes them hard, tough and strong.

Parenchyma cannot do this, because its cells are living, thin-walled, loosely packed, with large vacuoles. They are soft and suited to storage and food-making, not to strength.

Sclerenchyma, whose long, dead cells have thick lignified walls; parenchyma is living, thin-walled and soft, so it gives no such strength.

The statement is incorrect. Meristems are found in three places:

• Apical meristem at the tips of roots and shoots (increases length), which Vibha mentioned.
• Lateral meristem (cambium) along the sides of stems and roots (increases girth).
• Intercalary meristem at the base of leaves or internodes (e.g. grasses, bamboo, sugarcane), which helps them regrow.

Questions Neha could ask: "If meristems are only at the tips, how do tree trunks become thicker every year?" or "How does grass grow again after it is cut from the top, if the tip has been removed?"

Incorrect: meristems are also lateral (cambium, for girth) and intercalary (at the bases of internodes/leaves). Neha can ask, "Then how does a tree trunk grow thicker?" or "How does cut grass regrow?"

(i) The plant cell. A mature plant cell usually has one large central vacuole that can fill most of the cell; it stores cell sap and keeps the cell turgid, which gives rigidity. Animal cells have small, temporary vacuoles (or none).

(ii) Assumptions: that both are mature, typical cells (not young meristematic plant cells, which have tiny or no vacuoles); that the plant cell is a living parenchyma-type cell; and that the animal cell is a typical one, not a specialised cell (like a fat cell) that stores large droplets.

(i) The plant cell, which has a large central vacuole for storage and turgidity; animal cells have only small vacuoles. (ii) Assuming both are mature, typical (not meristematic or specialised) cells.

Questions to ask:

• Do any tissues carry out more than one function?
• Can the same tissue do different jobs in different parts of the plant?
• Do the different cell types in complex tissues have different roles?

Examples to examine:

• Parenchyma: stores food, carries out photosynthesis (as chlorenchyma in leaves), helps in buoyancy (aerenchyma in water plants), and fills spaces. So it has several functions.
• Collenchyma: gives both mechanical support and flexibility; in some plants it also does photosynthesis.
• Xylem: conducts water and minerals and gives mechanical strength (wood); its parenchyma stores food.
• Phloem: transports food, and its fibres give strength.
• Epidermis: protects and also helps in gas exchange (stomata) and water absorption (root hairs).

So the statement is not correct: many plant tissues perform more than one function.

Ask whether any tissue has more than one role and whether its role differs by location; examine parenchyma (storage, photosynthesis, buoyancy), collenchyma, xylem (conduction + strength), phloem and epidermis; this shows tissues often have several functions.